The present invention relates to apparatus for detecting the degree of wetness of sludge as it is being dewatered on a gravity belt thickener, belt press or the like and to controls, based on streaming current monitoring technology, for adjusting polymer dosage, on-line, in wastewater treatment processes.
In the prior art, optical devices, including a video camera focused on a fixed portion of a sludge dewatering belt, have been used to identify the exact moment at which the wetline on the belt crosses a given point. Typically, this point is located after the last row of dewatering plows; and the movement of the wetline into the camera's viewing range signals that there has been a sudden change-in process conditions causing the sludge to become wet.
Previously, the video camera has been used to track visually the onset of sludge dewatering systems washouts. When the sludge cake becomes wet, there are less distinct trails behind the plows; and the camera image becomes more black. Such washouts can be related to a certain distinctive pattern in which “camera readings” in processed video signals drop precipitously over time. Generation of such “camera readings” entails the use of a video monitoring system complete with a PC (personal computer), video input card, and suitable software, such as the Polytracker™ program available commercially from Chemtrac Systems, Inc. of Norcross, Ga. The latter software divides the pixels of the camera's image into 1 of 2 bins—black and white. A “pixel threshold” programmed into this software establishes the value by which the pixels are split into one or the other of the two bins. Calibration of the “camera reading” is achieved by adjusting the “pixel threshold,” setting until the “camera reading” registers around 50 (on a scale of 0 to 100) at the same time the sludge cake exhibits an optimum degree of dryness, exposing sizeable portions of the dewatering belt because of good sludge separation behind the plows.
Unfortunately, video image processing technology by itself is not sufficiently sensitive to be of use in controlling polymer dosage in dewatering systems automatically. A significant underfeed of polymer, for example, initially causes the “camera readings” to change only slowly. As the sludge dryness decreases and the sludge closes in behind the plows, the “camera readings ” first start to climb towards the upper 50's and 60's, still relatively close to the usual “camera reading” setpoint of 50. But if the belt completely washs out, the “camera readings” suddenly reverse direction and drop towards 0—the same as if the sludge had become excessively dry—even though the video image has gone all black and no white from the belt is showing through. In the case of a washout, this precipitous drop occurs because of an adjustment which a light compensation feature of the video camera makes, actually shifting the pixels being measured toward the lower (white) threshold. The resultant pattern while useful in helping an operator to initially set a minimum polymer dosage for a given set of process conditions, cannot be used to accurately detect, on-line, which direction polymer dosage may need to be adjusted in response to changing process conditions.
Conveniently, output from the streaming current monitor (SCM), modified with larger flow paths and an automated cleaning system to help prevent plugging and fouling of the SCM sensor in the wastewater environment, can be related to polymer dosage. Employing a pair of electrodes to detect a current which is generated when a solution of electrically charged particles is hydraulically sheared by a piston as it reciprocates within a cylinder on which the electrodes are mounted, as disclosed in U.S. Pat. Nos. 4,769,608 and 5,119,029, the SCM measures the net ionic and colloidal surface charge, whether positive or negative, in the sample being tested. Used to measure on-line the streaming current, or charge, of filtrate captured as the sludge is being dewatered, the streaming current monitor responds rapidly to changes in the filtrate composition. Most polymers used in sludge dewatering are positively charged (cationic), and most municipal wastewater solids requiring dewatering are negatively charged (anionic). If the SCM detects a higher than normal cationic charge in the filtrate, then it is likely that excess polymer is being fed.
In the prior art, signals from SCMs were fed to controllers to make changes in the polymer dosage so as to maintain the charge of the filtrate at a predetermined setpoint. It was left up to individual operators to determine both what the initial streaming current setpoint should be and when it required adjustment. In some situations, the high frequency of required setpoint adjustments—necessitated because of certain changes in sludge composition and even changes in flow rates, especially when short retention times were involved—led operators to question the effectiveness of using streaming current monitoring technology in wastewater treatment plants (WWTPs).